The present invention relates generally to air conditioners, and more particularly to a system and method for controlling the degree of superheat of an air conditioner that employs a pulse width modulated compressor.
With the enlargement of buildings, there is an increasing consumer demand for a multi-air conditioner in which a plurality of indoor units are connected to a single outdoor unit. In such a multi-air conditioner, the required cooling capacities of the indoor units vary according to change of the environmental characteristics of locations where the indoor units are installed. This requirement is fulfilled by adjusting the openings of electronic expansion valves positioned upstream of an indoor side heat exchangers or evaporators. In particular, in the case of a multi-air conditioner, since the lengths of the refrigerant conduits between its indoor units and outdoor unit are different and therefore refrigerant undergoes different flow resistances, the degrees of superheat of its evaporators (the degree of superheat of each evaporator=the exit temperature of the evaporatorxe2x88x92the entrance temperature of the evaporator) are different. In theory, the degree of superheat means the difference between the temperature of superheated vapor and the temperature of saturated vapor. Often, in practice, the difference between the entrance and exit temperatures of an evaporator is regarded as the degree of superheat, and is used to control the degree of superheat.
When the degree of superheat of refrigerant flowing out of an evaporator is high, superheat of a compressor and decrease in efficiency of the compressor are caused. When the degree of superheat is excessively high, a safety device is operated and the entire operation of an air conditioning system is stopped. On the other hand, when the degree of superheat is excessively low, the possibility that liquid refrigerant enters the compressor becomes high.
As a result, it is necessary to properly control the degrees of superheat of indoor side heat exchanges (evaporators) so that the performances of the indoor units are maximized, the difference between the performances of the indoor units are reduced and the efficiency and stability of the compressor and entire system are improved.
In the conventional multi-air conditioner, since a constant speed type or variable rotation number type compressor is employed, the flow rate of refrigerant does not vary largely over time during the operation of the compressor. Accordingly, since the entrance and exit temperatures of the evaporators smoothly vary as shown in FIG. 8, it is not difficult to control the degrees of superheat using the difference between the entrance and exit temperatures detected.
Pulse width modulated compressors are disclosed as another type of variable capacity compressors in e.g., U.S. Pat. No. 6,047,557 and Japanese Unexamined Pat. Publication No. Hei 8-334094. These compressors are utilized in refrigerating systems each having a plurality of freezing compartments or refrigerating compartments, and designed to be applied to short piping in which the length of a refrigerant conduit situated between a compressor and an evaporator is short. Consequently, these compressors cannot be applied to air conditioning systems for buildings that necessarily employ long piping and are given control environments different from those for the refrigerating systems. Further, in the prior art, there is not disclosed a control system and method for utilizing a pulse width modulated compressor in an air conditioner, particularly, in a multi-air conditioner, and in particular a method for controlling the degree of superheat.
When a pulse width modulated compressor is utilized in an air conditioner, the flow of refrigerant is periodically started and stopped because of a loading time, during which refrigerant is discharged, and an unloading time, during which refrigerant is not discharged, are periodically repeated even while the compressor is operated. Accordingly, an air conditioner employing a pulse width modulated compressor has the characteristic (a temperature fluctuation phenomenon) that the entrance and exit temperatures of the evaporator are varied up and down depending on the presence and absence of the flow of refrigerant. Hence, in an air conditioner employing a pulse width modulated compressor, the degree of superheat of an evaporator cannot be properly calculated by the entrance and exit temperatures of the evaporator detected at a given instance.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a system and method for controlling the degree of superheat of an air conditioner employing a pulse width modulated compressor, in which the degrees of superheat of evaporators corresponding to the actual degrees of superheat are calculated by compensating for the fluctuation of the entrance and exit temperatures and electronic expansion valves are controlled on the basis of the calculated degrees of superheat, thereby operating the entire air conditioning system at an optimum state.
In order to accomplish the above object, according to an aspect of the principles the present invention, a system for controlling the degree of superheat of an air conditioner comprises a compressor controlled in a pulse width modulation manner according to duty control signals; a condenser, an electronic expansion valve and an evaporator, constituting a refrigeration cycle, together with said compressor; means for sensing properties for calculating the degree of superheat of said evaporator; and a control unit for regulating opening of said electronic expansion valve according to the calculated degree of superheat.
According to another aspect of the principles of the present invention, a method for controlling the degree of superheat of an air conditioner, said air conditioner including a compressor controlled in a pulse width modulation manner according to duty control signals, an electronic expansion valve and an evaporator, comprising the steps of: detecting entrance and exit temperatures of said evaporator; calculating degree of superheat on the basis of detected entrance and exit temperatures; calculating target opening value of said electronic expansion valve on the basis of the calculated degree of superheat; and regulating said electronic expansion valve to a calculated target opening value.